For running on ice and snow on a road, use of a studded tire and fitting of chains on tires have been employed so far, and in order to cope with an environmental problem such as a problem with a dust caused thereby, a studless tire has been developed. In order to enhance low temperature property of a studless tire, various improvements have been made from material and design points of view, and for example, a rubber composition prepared by compounding a large amount of mineral oil to a diene rubber being excellent in low temperature property, or the like has been used. However, generally as an amount of mineral oil is increased, abrasion resistance is decreased.
On an ice- and snow-covered road, as compared with a normal road surface, a friction coefficient of a tire decreases significantly and slippage is apt to occur. Therefore, not only low temperature property but also well-balanced performance on ice and snow (grip performance on ice and snow) and abrasion resistance are demanded for a studless tire. However, in many cases, performance on ice and snow is inconsistent with abrasion resistance, and it is generally difficult to improve the both properties simultaneously.
In order to improve performance on ice and snow and abrasion resistance in good balance, there is a prior art (Patent Document 1) for blending silica and a softening agent in large amounts. However, there is still a room for improvement from the viewpoint of well-balanced improvement of the both performances.
Further, a method of compounding a plurality of polymer (rubber) components (polymer blend) has been employed as a method of improving various tire performances such as low temperature property, performance on ice and snow and abrasion resistance in good balance. Specifically, a mainstream of the method is to blend some polymer components represented by a styrene-butadiene rubber (SBR), a butadiene rubber (BR), and a natural rubber (NR) as rubber components for a tire. This is a means for making good use of characteristic of each polymer component and deriving physical properties of a rubber composition which cannot be derived only by a single polymer component.
In this polymer blend, a phase structure (morphology) of each rubber component after vulcanization and a degree of distribution (localization) of a filler into each rubber phase will be important factors for deciding physical properties. Elements for deciding control of morphology and localization of a filler are very complicated, and various studies have been made in order to exhibit physical properties of a tire in good balance, but there is a room for improvement in any of the studies.
For example, Patent Document 2 discloses a technology of specifying a particle size of an island phase and a silica distribution in an sea-island matrix of a rubber composition for a tire tread comprising a styrene-butadiene rubber. However, regarding a concrete method enabling the morphology thereof to be realized, there are described only use of a master batch comprising silica and adjustment of a kneading time and a rotation torque of a rotor, and in such a method, the morphology is affected greatly by kneading and vulcanizing conditions, and therefore, stable control of the morphology is difficult. Further, the rubber component disclosed in examples is a combination of styrene-butadiene rubbers having relatively similar polarities. Therefore, it is apparent that the disclosed technology cannot be applied to the blending of rubber components having greatly different polarities, namely greatly different affinities for silica such as blending of a butadiene rubber and a natural rubber.
Particularly in the case of control of dispersion of silica between the phases using a master batch comprising silica, even if a desired morphology and silica dispersion are achieved temporarily, in many cases, the morphology and the silica dispersion change with a lapse of time and therefore, it was difficult to form a morphology being stable with a lapse of time of more than several months.
Patent Document 3 discloses a technology relating to control of a morphology and localization of silica in a compounding formulation comprising a natural rubber and a butadiene rubber. However, there is no description regarding the control of localization of silica into the butadiene rubber side in the case where the butadiene rubber which is disadvantageous to localization of silica forms a continuous phase.
A natural rubber is an important rubber component in a rubber composition for a tire, especially for a side wall because of its excellent mechanical strength, etc. However, in the case of blending with a butadiene rubber, localization of silica is apt to arise, and it is necessary to set the compounding formulation while controlling a distributing state of silica. However, so far a morphology and a distribution state of silica have not been checked sufficiently, and there was a case of a compounding formulation giving an insufficient exhibition of physical properties.
Furthermore, recently, there is a tendency of conducting modification of a natural rubber for enhancing affinity thereof for silica in order to aim at enhancement of fuel efficiency, which makes possibility of localization of silica into a natural rubber more significant.
Further, recently there are many cases of compounding a high cis butadiene rubber being excellent in abrasion resistance and low temperature grip performance. However, among diene rubbers, a high cis butadiene rubber is low in affinity particularly for silica and in a compounding system thereof with a natural rubber, there is a tendency that silica is hardly incorporated into a high cis butadiene rubber phase. Therefore, in a conventional system of compounding a high cis butadiene rubber, in some cases, a compounding formulation so as not to exhibit sufficient physical properties was employed while a morphology and a state of silica distribution were not confirmed.
In particular, in a rubber composition for a side wall, it is important to prepare a rubber composition comprising, as a continuous phase, a butadiene rubber having performance required for a side wall such as flex-crack resistance, and a technology of conducting control of silica localization on a continuous phase rubber component making a great contribution to abrasion resistance is essential.
Further, a natural rubber tends to hardly form a continuous phase as compared with a butadiene rubber, and in a compounding system where a natural rubber is blended in an amount of not more than 50 parts by mass based on 100 parts by mass of rubber components, such a tendency is further significant, and a so-called island phase is formed. Generally, a circumference of a rubber component being present in an island phase is solidified with a rubber component of a continuous phase, and therefore, there is a tendency that a hardness of the rubber component of an island phase increases and a rubber elasticity thereof is lowered. If a filler is localized, the tendency thereof increases more, and as a result, a difference in a hardness from the continuous phase rubber component increases, thereby easily causing a decrease in a rubber strength and abrasion resistance. A natural rubber is apt to have a hardness larger than a butadiene rubber even in the case of a single use thereof, and therefore, primarily it is not desirable that a difference in hardness further increases due to localization of silica. Therefore, development of a technology of not causing excessive localization of silica at a natural rubber side is important.
With respect to formation of a morphology of a plurality of polymer components in a rubber composition for a tire, only a compatible type (single phase) or in the case of incompatible type, only an sea-island phase structure, wherein a phase (island phase) of other particulate component is present in a continuous phase (sea phase), have been studied so far.
Therefore, in a system using a blend of a butadiene rubber and a natural rubber which is useful for exhibiting physical properties of a tire while polarities thereof are different from each other, development of technologies for morphology control and silica distribution to exhibit good physical properties of a rubber has been considered to be necessary.